The present invention relates generally to printing equipment and is particularly directed to ink jet printers of the type which uses multi-pass printing, called shingling, to form bitmap images of full intended coverage. The invention is specifically disclosed as a shingle mask that is derived from a shingle mask density distribution which exhibits a substantially trapezoidal shape, and thereby reduces banding effects by effectively increasing a number of printed-density bands which are correspondingly decreased in size, while at the same time not increasing the number of printhead passes over a given area on the print media, and thus not negatively impacting printed throughput.
Banding is currently the primary defect in ink jet printing. Without banding, existing ink jet printing technology can easily achieve quality comparable to conventional photography. Typically, ink jet printers approach xe2x80x9cphoto qualityxe2x80x9d by using multi-pass printing. As the name implies, such printing makes multiple passes of the printhead, rather than the ordinary single pass printing. Each printing pass sub-samples the image by using a special xe2x80x9cshingle maskxe2x80x9d or xe2x80x9cprint mask.xe2x80x9d The sub-sampling, or xe2x80x9cshingle mask,xe2x80x9d distributes the location errors of the individual ink drops caused by nozzle or nozzle firing abnormalities or other system errors. Such misplaced drops are blended with other normal ink drops, making the misplaced drops more difficult to detect. Multi-pass printing also increases the banding frequency, which makes the banding less visible and less objectionable to human visual systems. Therefore, the larger number of passes made using the multi-pass process, the better the print quality can be. However, increasing the number of passes involves a substantial penalty in throughput.
Various methods for designing a shingle mask to xe2x80x9caverage outxe2x80x9d printing defects and to suppress banding are disclosed in existing patent documents. For example, Hewlett-Packard owns a number of patents involving using some type of print mask to reduce print artifacts, including banding-type artifacts. One of these patents is U.S. Pat. No. 5,992,962 (by Yen), which discloses a print mask used for ink jet printers that is designed to reduce print artifacts, both to reduce banding and print ink migration. The Yen invention reduces the banding by using multi-pass printing (also known as xe2x80x9cshinglingxe2x80x9d), and states that the earlier prior art print masks had provided checkerboard patterns. In Yen, the print mask provides triangular clusters that are complimentary from the first pass to the second pass of printing. The primary example of the triangular clustered patterns used in Yen is illustrated on FIG. 6 (of Yen), in which the top row of one print pass is all dots, while that same top row in the second pass would be all non-dots. In the first pass, the top row is divided up into 4xc3x974 tiles, and the dots in the top row represent the base of the triangle (per tile). Yen describes the complimentary print masks as being asymmetric, and also provides the benefits of turning off one of the top or bottom nozzles in each of the passes, which further helps to reduce banding artifacts. In addition to the above reduction of banding artifacts, the Yen patent describes xe2x80x9cmufflingxe2x80x9d one or more nozzles of a first mask matrix in situations where a defective nozzle is determined, and that nozzle is then disabled in the first print pass. Then a complimentary nozzle is enabled in the second print pass to print all of the dots that would have been printed in the first pass by the defective nozzle.
U.S. Pat. No. 6,213,586 (by Chen, also owned by Hewlett-Packard) discloses an ink jet printer that produces temporally or spatially shingled images for a multicolor printhead. The example in the Chen patent is for a six-color ink jet printer, in which there are two different shades each of cyan and magenta. Each color has a xe2x80x9cdeposition maskxe2x80x9d that comprises a matrix of threshold values, and each color has a set of xe2x80x9cshingle control values.xe2x80x9d The deposition masks allow for both temporal and spatial shingling to occur during successive scans of the printhead so as to avoid or reduce image artifacts. The deposition mask is tiled on the bitmap, and the shingle control value set for each color determines whether or not a particular colored dot will be placed on a particular scan (or pass) of the printhead. The only clear example as to how this spatial or temporal shingling is supposed to reduce image artifacts is described on column 4 starting at line 46, where it states that the threshold values in each deposition mask are arranged to assure that the color intensities and amounts of ink deposited at swath extremities xe2x80x9cclosely matchxe2x80x9d between succeeding swaths.
U.S. Pat. No. 4,999,646 (by Trask, also owned by Hewlett-Packard) discloses a method for enhancing the uniformity and consistency of ink jet dot formation. This patent uses multiple pass complimentary dot patterns to minimize many undesirable characteristics, including coalescing, beading, and color bleed. Trask uses a partial overlap between multiple passes that use complimentary dot patterns. The dot spacing in coincident dot rows within the overlapped portions is alternated between dots of the first and second patterns. In addition to the above, Trask uses an xe2x80x9cimproved dot-next-to-dotxe2x80x9d super pixeling to further optimize ink drop drying conditions to produce optimized uniformity and consistency of dot formation. The partial dot overlap process alleviates print quality problems in three ways: (1) a 50% checkerboard or other overlap pattern is chosen to minimize interactions between individual drops while they are drying; (2) the 50% dot pattern overlap of two swaths breaks up the horizontal drying patterns and minimizes banding; and (3) the use of alternating nozzles in the overlap dot rows minimizes the impact of nozzle variations.
Another U.S. patent in this area is owned by Colorspan Corporation, U.S. Pat. No. 5,790,150 (by Lidke), which discloses a method for controlling an ink jet printer in multipass printing. The pixel locations for each pass are controlled so that no pixel on a particular pass is orthogonally or diagonally adjacent to any other pixel location that is to be printed in the same pass. At least four passes are made on the print media before the printhead is advanced to a new swath. This also means that no pixel (dot) location is immediately adjacent to any other pixel (dot) location being printed on that pass.
Many conventional ink jet printers use a swath-by-swath approach and this approach typically causes various defects to appear periodically across the page, and is commonly known as the xe2x80x9cbanding defect.xe2x80x9d There are typically two types of density variations that comprise banding defects: high-frequency variations and low-frequency variations. Hi-frequency errors are generally caused by location errors in the placement of individual ink drops, probably originated from the printhead. Low-frequency density errors have a variety of causes, including halftone moirxc3xa9 patterns, alignment errors, and color difference caused by ink printing order changes. High-frequency density variations are the subject of conventional shingle mask designs. However, low-frequency density variations contribute substantially to the overall banding defects present in high-throughput printing methods that reduce the number of passes. The present inventors have conducted a study from which it was found that the human visual system is more sensitive to square wave variations than to other types of smooth variations of the same density contrast. It would therefore be advantageous to produce a smooth xe2x80x9cbanding profile,xe2x80x9d that will tend to reduce the visibility of banding defects.
Multi-pass printing on carriage-based printing technologies (e.g., used for ink jet printers) helps to relieve print defects created by mechanical tolerances such as banding and pel location error. Currently a bitmap is divided into swathes of information. Each swath contains a portion of the bitmap vertically, and also the entire width of the bitmap across the page. The vertical size of each swath is maximally the size of the ink jet printhead, but it may be smaller. When printing in a single-pass mode, the distance between the top scanline of a swath and the swath following is equal to the height of the swath. However, when printing in multi-pass modes, the distance between the top scanline of a swath and the swath following can be any distance less than the height of the swath. An example would be four-pass printing.
In a four-pass printing mode with a swath height of 320 (i.e., the number of nozzles on the printhead), the typical distance between the top of each successive swath would be 80 scanlines. In a 4-pass printing mode, each pel location will be passed over by a nozzle four times. For this reason, it is important to apply a print mask (or shingle mask) to each printed swath which prevents a pel from receiving more than the desired amount of ink.
In conventional ink jet printers, shingle masks have traditionally had a uniform density distribution, consisting of either ordered or unordered random pel arrangements. These masks have helped to hide pel-location errors and banding problems. The unordered variety has helped to hide pel-location errors somewhat better, similar to error-diffusion techniques. However, both types of masks do not eliminate the banding problems, and significant improvements can be made. A problem with a uniformly distributed mask is the abrupt transitions or steps in the xe2x80x9caccumulated density.xe2x80x9d
The accumulated shingle mask distribution consists of the apparent steps seen while an ink jet printer is printing. For a conventional four-pass system, the accumulated distribution of a uniformly distributed shingle mask is depicted in FIG. 1. The X-axis represents the number of scanlines, while the Y-axis represents the numbers of drops printed. (This represents a 1-bit shingle mask.) The number of accumulated drops changes drastically every time another group of nozzles is used for the xe2x80x9cnextxe2x80x9d pass. The nozzle groupings are indicated by the designations G1, G2, G3, and G4.
During the first pass, the accumulated drops will be at the level indicated by the reference numeral 10. During the second pass, the number of drops will suddenly rise to the level indicated by the reference numeral 12. During the third pass, the number of accumulated drops printed will rise to the level indicated by the reference numeral 14. Finally, the number of drops printed will rise to the xe2x80x9cintended coverage levelxe2x80x9d which is indicated by the reference numeral 16.
As can easily be seen, there are abrupt transitions in the accumulated density, mainly because the shingle mask has a uniform distribution pattern. The result of this is illustrated in FIG. 2, which depicts the accumulated banding pattern as the passes are made. As noted above, FIGS. 1 and 2 are representative of a 4-pass shingling system, in which the first pass prints the scanlines (or nozzle numbers) in the range of 241-320, while the second group or pass represents the scanlines (nozzles) in the range of 161-240, the third xe2x80x9cbandxe2x80x9d represents the scanlines (nozzles) in the range of 81-160, while the fourth pass or band represents the scanlines (nozzles) in the range of 1-80. As can be seen from FIG. 2, there is a very distinct banding pattern, as illustrated in the accumulated pass bands indicated by the reference numerals 20, 22, 24, and 26.
Even after the printing process has been completed, remnants of these shingle mask steps may be visible. The leftover remnants comprise the defect known as banding. The sharp transition regions are caused by variations in drying time, bleeding characteristics, forms advance error, and other reasons. Depending on the banding frequency, the defects may or may not be detectable by the human eye. In this example, a noticeable banding transition region is present every eighty scanlines, and if the printing resolution is 600 dpi (dots per inch), then the banding transition region is present every 80/600 inches.
As described above, uniformly distributed shingle masks tend to have banding problems due to the sharp transition regions present after each pass of the printhead. It would be an improvement in the banding characteristics of an ink jet printer to eliminate the sharp transition regions.
Accordingly, it is an advantage of the present invention to eliminate sharp transition regions to assist in removing banding problems. It is another advantage of the present invention to provide a shingle mask that creates a smooth accumulated shingle mask distribution, which will tend to eliminate the sharp transition regions that create the noticeable banding characteristics of ink jet printers. It is a further advantage of the present invention to provide a shingle mask derived from a shingle mask density distribution that is substantially trapezoidal in shape.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a method for reducing ink jet printer banding effects is provided, in which the method comprises: (a) selecting a profile for an accumulated shingle mask distribution, wherein the profile: (i) exhibits a substantially flat horizontal shape in a first portion, and (ii) exhibits a substantially smooth decreasing shape in a second portion; (b) quantizing the accumulated shingle mask distribution, and deriving at least one shingle profile corresponding to the quantized accumulated shingle mask distribution, wherein the quantized accumulated shingle mask distribution includes a third portion and a fourth portion which correspond, respectively, to the first and second portions of the accumulated shingle mask distribution; (c) deriving a shingle mask density distribution using the at least one shingle profile; and (d) deriving a shingle mask corresponding to the shingle mask density distribution.
In accordance with another aspect of the present invention, a method for generating a shingle mask used in a printer capable of printing drops in a plurality of printing passes, in which the method comprises: (a) selecting initial constraints, including: (i) a mask height and width, (ii) a number of drops per pel, and (iii) a number of printing passes corresponding to full intended coverage; (b) selecting a shape of a banding profile, in which the shape comprises a plateau portion and a substantially smooth decreasing portion; (c) selecting a height of the plateau portion; (d) quantizing the banding profile, thereby deriving a quantized banding profile, and determining a plurality of shingle profiles, wherein a plateau portion of the quantized banding profile has a horizontal length that significantly exceeds a horizontal length of any of a plurality of discrete levels of the derived quantized banding profile; (e) based upon the plurality of shingle profiles, deriving a shingle mask density distribution; and (f) based upon the shingle mask density distribution, generating a shingle mask of the mask height and width.
In accordance with a further aspect of the present invention, a shingle mask used in a printer capable of printing drops in a plurality of printing passes, in which the shingle mask comprises: a bitmap pattern of a predetermined sequence having a mask height and width, the bitmap pattern being derived from a shingle mask density distribution which exhibits a substantially trapezoidal, but not rectangular, shape on a graph in which an X-axis represents drops printed and a Y-axis represents nozzle positions on a printhead.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.